In any Universal Mobile Telecommunications System (UMTS) system frequency errors are introduced by clock mismatch between the user equipment and the base station and in the channel from Doppler effect. The user equipment tries to compensate for these errors by using Automatic Frequency Correction (AFC), which contains two main parts, i.e. frequency error estimation and error correction. In its simplest form, frequency error estimation can be performed by comparing received Common Pilot Channel (CPICH) pilots in the receiver with the sent ones. A frequency error will show up as a rotation of the recovered pilot symbols in the IQ plane over time.
The frequency error is normally composed of two parts; i.e. the frequency error due to the clock mismatches between the user equipment and the base station, which is also called frequency drift, and the random component due to Doppler effect. The first is slowly varying mainly because of temperature changes in the user equipment and the latter is a result of several impinging waves received at the user equipment. The angle of arrival and the speed which the user equipment travels determine the frequency shift for each wave.
In order to correct for frequency errors it is desired that both components be known independently. In single carrier systems there is no easy way to distinguish between these two sources of frequency error. The Doppler frequency can be estimated using multiple techniques none of which are very accurate or fast. Some of the well known estimation techniques include (but are not limited to), Level Crossing Rate (LCR), Zero Crossing Rate (ZCR) and Covariance-based estimators that exploit autocovariances of powers of the signal envelope.
The existing solutions use time domain characteristics like level crossing rate, which requires an appreciable measurement time and thus cannot react to fast Doppler changes. The Doppler frequency estimated is inherently noisy and thus needs to be used with some kind of filtering or hysteresis. This causes problems when there are frequent speed changes and the Doppler frequency estimation never converges. The current algorithms do not exploit the diversity in dual Cell High-Speed Downlink Packet Access (HSDPA) scenarios.
The Doppler component is adding noise to the frequency estimation. This leads to decreased accuracy in the frequency error estimation and requires strong filtering. Current Doppler frequency estimation techniques are very inaccurate and require a large number of samples and filtering to converge. In wideband systems where it is possible to resolve independent paths the present solutions cannot be used to estimate and compensate for the Doppler effect per path, i.e. per finger.
Multi-carrier operation involves jointly scheduling two or more HS carriers to increase the peak data rates per user and increase the utilization of available frequency resources by multiplexing carriers in CELL DCH state. Dual-carrier HSDPA is included in Release 8 of 3GPP, other variations such as Dual Band HSDPA, DC-HSDPA etc are included in Release 9. Release 10 includes 4 HS carriers and Release 11 is expected to include 8 HS carriers in two different bands.
In Dual-Carrier (or Multi-Carrier) HSDPA there are two types of carriers. The first carrier known as the ‘Anchor carrier’ carries all the legacy physical channels including DPCH/F-DPCH, E-HICH, E-RGCH, E-AGCH, PICH, SCCPCH, AICH etc. The other carriers are ‘supplementary’ carriers and carry a reduced set of physical channels in order to reduce signaling overhead.
A method of frequency tracking and compensation for frequency error due to Doppler effect in a plurality of multi-path signals is shown in U.S. Pat. No. 6,608,858. The Doppler compensation is based on only one carrier. Each finger of a RAKE receiver computes a frequency error for that finger, and a weighted average of all these frequency errors is calculated and filtered to provide a control signal for the internal frequency synthesizers. This method will not be able to separate the Doppler component and the drift component completely, since after averaging over fingers the error will still contain some sort of weighted average Doppler component, due to the limitation of using only one carrier.